Traditional approaches to cancer treatment suffer from a lack of specificity. Most drugs that have been developed are natural products or derivatives which block enzyme pathways or randomly interact with DNA. Moreover, most cancer treatment drugs are accompanied by serious dose-limiting toxicities due to low therapeutic indices. For example, the majority of anti-cancer drugs when administered to a patient kill not only cancer cells but also normal, non-cancerous cells. Because of these deleterious effects, treatments that more specifically affect cancerous cells are needed.
It has been found that a class of genes, the oncogenes, are involved in the transformation of cells, and in the maintenance of a cancerous state. Notably, disrupting the transcription of these genes, or otherwise inhibiting the effects of their protein products, can have a favorable therapeutic result. The role of oncogenes in the etiology of many human cancers has been reviewed in Bishop, 1987, “Cellular Oncogenes and Retroviruses,” Science, 235:305-311. In many types of human cancers, a gene termed bcl-2 (B cell lymphoma/leukemia-2) is overexpressed, and this overexpression may be associated with tumorigenicity (Tsujimoto et al., 1985, “Involvement of the bcl-2 gene in human follicular lymphoma”, Science 228:1440-1443). The bcl-2 gene is thought to contribute to the pathogenesis of cancer, as well as to resistance to treatment, primarily by prolonging cell survival rather than by accelerating cell division.
The human bcl-2 gene is implicated in the etiology of certain leukemias, lymphoid tumors, lymphomas, neuroblastomas, and nasopharyngeal, prostate, breast, and colon carcinomas (Croce et al., 1987, “Molecular Basis Of Human B and T Cell Neoplasia,” in: Advance in Viral Oncology, 7:35-51, G. Klein (ed.), New York: Raven Press; Reed et al., 1991, “Differential expression of bcl-2 protooncogene in neuroblastoma and other human tumor cell lines of neural origin”, Cancer Res. 51:6529-38; Yunis et al., 1989, “Bcl-2 and other genomic alterations in the prognosis of large-cell lymphomas”, N. Engl. J. Med. 320:1047-54; Campos et al., 1993, “High expression of bcl-2 protein in acute myeloid leukemia is associated with poor response to chemotherapy”, Blood 81:3091-6; McDonnell et al., 1992, “Expression of the protooncogene bcl-2 and its association with emergence of androgen-independent prostate cancer”, Cancer Res. 52:6940-4; Lu et al., 1993, “Bcl-2 protooncogene expression in Epstein Barr Virus-Associated Nasopharyngeal Carcinoma”, Int. J. Cancer 53:29-35; Bonner et al., 1993, “bcl-2 protooncogene and the gastrointestinal mucosal epithelial tumor progression model as related to proposed morphologic and molecular sequences”, Lab. Invest. 68:43A). Bcl-2 has been found to be overexpressed in a variety of tumors including non-Hodgkin's lymphoma, lung cancer, breast cancer, colorectal cancer, prostate cancer, renal cancer and acute and chronic leukemias (Reed, 1995, “Regulation of apoptosis by bcl-2 family proteins and its role in cancer and chemoresistance”, Curr. Opin. Oncol. 7:541-6).
Antisense oligonucleotides provide potential therapeutic tools for specific disruption of oncogene function. These short (usually less than 30 bases) single-stranded synthetic DNAs have a sequence complementary to pre-mRNA or mRNA regions of a target gene, and form a hybrid duplex by hydrogen-bonded base pairing. This hybridization can disrupt expression of both the target mRNA and the protein which it encodes, and thus can interfere with downstream interactions and signaling. Since one mRNA molecule gives rise to multiple protein copies, inhibition of the mRNA can be more efficient and more specific than causing disruption at the protein level, e.g., by inhibition of an enzyme's active site.
Synthetic oligodeoxynucleotides complementary to mRNA of the c-myc oncogene have been used to specifically inhibit production of c-myc protein, thereby arresting the growth of human leukemic cells in vitro (Holt et al., 1988, Mol. Cell Biol. 8:963-73; Wickstrom et al., 1988, Proc. Natl. Acad. Sci. USA, 85:1028-32). Oligodeoxynucleotides have also been employed as specific inhibitors of retroviruses, including the human immunodeficiency virus (Zamecnik and Stephenson, 1978, Proc. Natl. Acad. Sci. USA, 75:280-4; Zamecnik et al., 1986, Proc. Natl. Acad. Sci. USA, 83:4143-6).
The use of antisense oligonucleotides, with their ability to target and inhibit individual cancer-related genes, has shown promise in preclinical cancer models. These phosphorothioate antisense oligomers have shown an ability to inhibit bcl-2 expression in vitro and to eradicate tumors in mouse models with lymphoma xenografts. Resistance to chemotherapy of some cancers has been linked to expression of the bcl-2 oncogene (Grover et al., 1996, “Bcl-2 expression in malignant melanoma and its prognostic significance”, Eur. J. Surg. Oncol. 22(4):347-9). Administration of a bcl-2 antisense oligomer can selectively reduce bcl-2 protein levels in tumor xenografts in laboratory mice (Jansen et al., 1998, “bcl-2 antisense therapy chemosensitizes human melanoma in SCID mice”, Nat. Med. 4(2):232-4). Moreover, administration of a bcl-2 antisense oligomer can make tumor xenografts in laboratory mice more susceptible to chemotherapeutic agents (Jansen et al., 1998, “bcl-2 antisense therapy chemosensitizes human melanoma in SCID mice”, Nat. Med. 4(2):232-4). In mice, systemic treatment with a bcl-2 antisense oligomer reduced bcl-2 protein and enhanced apoptosis. Treatment with bcl-2 antisense oligomer alone had modest antitumor activity, but enhanced antitumor activity was observed when combined with DTIC (also known as dacarbazine). In ten of thirteen animals, no malignant melanoma xenografts were detectable after administration of bcl-2 antisense oligomer in combination with DTIC treatment. There remains a compelling need to extend these antitumor treatments to combat cancer in humans.
The prognosis of many cancer patients is poor despite the increasing availability of biologic, drug, and combination therapies. For example, although DTIC is commonly used to treat metastatic melanoma, few patients have demonstrated long-term improvement. In fact, an extensive phase III clinical trial did not demonstrate any better survival when DTIC was used in combination with cisplatin, carmustine, and tamoxifen (Chapman et al., 1999, “Phase III multicenter randomized trial of the Dartmouth regimen versus dacarbazine in patients with metastatic melanoma”, J. Clin. Oncol. 17(9):2745-51). These serious shortcomings in cancer treatments emphasize the need for new treatment approaches.